1. Field of the Invention
The present invention relates to a noise reduction device, and more particularly relates to a noise reduction device used in an enclosed structure such as an aircraft and a railroad vehicle.
2. Background Art
In an aircraft, a railroad vehicle, and the like, having a loud noise, when information provision such as announcement service to passengers sitting in seats is carried out, noises at their seats are a problem.
Interior space such as an aircraft and a railroad vehicle whose boundary is made by a continuous wall is a kind of an enclosed structure. When such space includes a noise source inside and outside thereof, users are seated in an environment in which a noise is fixed. Therefore, depending on the degree of noise, the noise may be a factor of physical and mental pressure to users, thus reducing comfort for users. In particular, when such space is used as a cabin in an aircraft and services are provided to passengers in the space, the quality of services may be seriously affected.
In particular, main noise sources of an aircraft include a noise of devices such as a propeller and an engine for generating thrust of an aircraft, a noise associated with airflow, for example, a wind noise during flight, which is generated when a aircraft body moves in air space. Noises inside an aircraft make passengers uncomfortable and hinder announcement service, and the like, and therefore improvement is demanded.
In response to this, as measures to reduce noises in an enclosed room, conventionally, a method by a passive attenuator is generally employed. A noise insulation material having a sound-absorption property such as a barrier material and an absorption material is disposed between an enclosed structure and a noise generation source. An example of the barrier material includes a high-density barrier material, and an example of the absorbing material includes a sound-absorption sheet. A material having a sound-absorption property has generally high density and a high-density material increases weight. When the weight increases, a flight fuel is increased and thus flying range is reduced. Therefore, cost efficiency and function as an aircraft are caused to be reduced. Furthermore, in a structure material, deterioration in strength such as damageability and functional deterioration in design such as deterioration of feeling of quality are not negligible.
In order to address the above-mentioned problems of the noise-reduction measure by a passive attenuator, as a method for reducing a noise by an active attenuator, a method for generating an acoustic wave having the phase opposite to the phase of a noise is generally carried out conventionally. With this method, a noise level in the noise generating source or the vicinity thereof can be reduced, and a noise can be prevented from propagating to a region that needs reduction of noise. Specifically, a sound cancellation device including a microphone for detecting a sound emitted from a noise source, a controller for amplifying an electrical signal input from the microphone and reversing the phase; and a loudspeaker for converting the electrical signal input from the controller into a sound and transmitting the sound is proposed.
In an aircraft and the like, based on the method for reducing a noise with the active attenuator mentioned above, noise-reduction measure is carried out from the viewpoint of improving comfort of a passenger seat. For example, a method for placing a noise-reduction device in every seat, and installing a loudspeaker, a microphone and a controller in the vicinity of the seat; and a method for reducing a noise in space by disposing a plurality of loudspeakers and microphones in the vicinity of a user sitting in a seat have been proposed (see, for example, patent documents 1 and 2).
Furthermore, in an aircraft and the like, in order to provide service in which comfort in a seat is enhanced, shell-structured seats each of which is surrounded by a structure are provided in a part of a cabin, and the seats are partitioned from each other by a structure, so that a noise entering from the surrounding of the seat is suppressed.
As an example of a method for enhancing the effect of suppressing a noise by using both a structure and a noise reducing device, a technology about a position relation between a sound-insulating wall and a noise detection unit is conventionally disclosed (see, for example, patent documents 3 to 5).
However, in service providing a high comfort, for example, a shell-structured passenger seat in an aircraft, high quality as to the noise level of the seat is also demanded. An aircraft in which many seats and devices are placed has a very complicated noise environment, so that the noise reduction devices installed at the seats are required to have a performance and quality corresponding to such a noise environment. In an aircraft, various noise sources from low frequency to high frequency are present. When a noise detection microphone picks up voices emitted from passengers of the aircraft and noises emitted from devices to be used, such noises become a noisy sound, so that a sufficient noise reduction effect cannot be obtained. To such problems, a conventional technology has paid attention to the relation between a noise detection microphone and an error microphone for detecting a remaining noise at a control point in space that is divided into two by a structure as a boundary for reducing a noise. However, a method for disposing the noise microphone in the above-mentioned two spaces has not been considered. Therefore, there is a problem that a conventional technology is used as a technology for realizing a high quality effect of reducing noises in particular place like seats in an aircraft and the like.    [Patent document 1] Japanese Patent Unexamined Publication No. H5-289676    [Patent document 2] Japanese Patent Unexamined Publication No. H5-281980    [Patent document 3] Japanese Patent Unexamined Publication No. H9-034472    [Patent document 4] Japanese Patent Unexamined Publication No. H7-020880    [Patent document 5] Japanese Patent Unexamined Publication No. H5-158485